This invention relates generally to a cooling system for a primary containment vessel (PCV) in a nuclear power plant, and more particularly to a passive cooling system with no active components (e.g. pumps) which system has an improved performance.
Passive cooling systems for a primary containment vessel in a nuclear power plant are less subjected to a malfunction because they do not employ active components such as pumps, and therefore are high in reliability.
One type of such passive cooling system is a condenser-type heat removal system disclosed, for example, in Japanese Patent Unexamined Publication Nos. 4-98198 and 4-136794.
In such a cooling system, a cooling water pool is provided outside a pressure boundary of a primary containment vessel, and a condenser (heat exchanger) is provided within this cooling water pool, which condenser is connected to a main steam line from the Reactor Pressure Vessel or to a gas-phase space in the primary containment vessel.
In this technique, when a loss of coolant accident (LOCA), which must be taken into consideration in designing the Nuclear Power Plant, occurs, steam produced in a reactor pressure vessel is led to the condenser, and is condensed there. At this time, uncondensed steam which has not been condensed in the condenser is discharged, together with noncondensable gas introduced into the condenser from the gas-phase space of the primary containment vessel, into a suppression pool within the primary containment vessel through a gas vent pipe, and this uncondensed steam is condensed in the water of this suppression pool such that the heat produced by this condensation is accumulated as sensible heat in the suppression pool. Thus, a pressure increase of the primary containment vessel at the time of an accident is suppressed.
With respect to this condenser-type heat removal system, a thermal-hydraulic behavior by the uncondensed steam discharged into the suppression pool through the gas vent pipe is described in "Proc. of Fifth International Topical Meeting on Reactor Thermal Hydraulics, Vol II (September 1992) pages 547-555".
Japanese Patent Unexamined Publication No. 63-1995 discloses another conventional arrangement in which there is provided a steam discharge device for discharging steam from a reactor pressure vessel into a pool water via a discharge pipe (vent pipe) and a safety relief valve, and a discharge portion of the discharge pipe (vent pipe) is constituted by a horizontal pipe having a plurality of holes or opening through which the steam is discharged into the pool water. The steam is discharged from the discharge holes in a fine manner, and the discharge holes are dispersed at such a density that steam bubbles from the adjacent holes can not easily be combined together.
The above Japanese Patent Unexamined Publication No. 63-1995 does not describe the combination of this steam discharge device with a condenser-type heat removal system.
In the conventional condenser-type heat removal system, a non-condensable gas, charged during a normal operation in the gas-phase space in the primary containment vessel surrounding the reactor pressure vessel, flows together with steam, into a condenser at the time of an accident when a cooling system is operated, thereby degrading the condensation capability.
In order to deal with the uncondensed steam which has not been condensed as a result of the degraded condensation capability, there is provided a gas vent pipe extending from a heat exchanger (condenser) into a suppression pool to introduce the uncondensed steam into the water in the suppression pool for condensation.
In the prior art technique, however, heat of the uncondensed steam discharged into the water of the suppression pool is presumed to be absorbed as sensible heat of the pool water at a depth to which the gas vent pipe is submerged, and no consideration has been given to a thermal-hydraulic behavior in the water of the suppression pool.
Referring to the thermal-hydraulic behavior in the suppression pool described in the above-mentioned literature, the uncondensed steam discharged from the gas vent pipe into the suppression pool is condensed by the water of this pool.
However, since an amount of the uncondensed steam flowing into the suppression pool is small, condensation is finished after only a part of the pool water disposed in the close vicinity of the outlet of the gas vent pipe is made hot. The hot water produced by this condensation moves upward to the suppression pool surface while forming a thin thermal boundary layer (having a thickness of about 10.about.15 cm) along the gas vent pipe.
Moreover, the region where the water is made hot in the vicinity of the outlet of the gas vent pipe is small, and therefore the volume of that portion of the suppression pool water which produces buoyancy by the high temperature is small. As a result, flow can not be induced in the suppression pool water, and the bulk water except for vicinity of the gas vent pipe remains stagnant.
Furthermore, in the suppression pool water, there is not provided any cooling means for inducing a downward flow from the water surface of the suppression pool. Therefore, the hot water rising along the gas vent pipe will not be mixed with the bulk water in the suppression pool.
Namely, since the condensation region is small, a relatively great temperature rise of the suppression pool water due to the condensation occurs in a localized portion of the suppression pool, and the hot water moves upward to the surface of the suppression pool water without being sufficiently mixed with the surrounding bulk and the hot water is accumulated in the vicinity of the surface of the pool water upon arrival at this water surface.
As a result, in the event of an accident, only an upper layer of the suppression pool water near to the water surface thereof is made hot. In other words, the water of the suppression pool is not sufficiently effectively used as a heat absorption source.
Under the circumstances, the following problems arise in view of a pressure behavior of the primary containment vessel at the time of an accident, as well as the strength of this containment vessel which should withstand this pressure behavior.
Namely, if only the upper layer of the suppression pool water near to the water surface thereof becomes hot as described above, this hot water causes the temperature of the gas-phase space (wetwell) at the upper portion of the suppression pool to rise, thereby increasing a steam partial pressure in this space.
The pressure within the primary containment vessel at the time of an accident corresponds to the sum of the partial pressure of the noncondensable gas and the steam partial pressure in the wetwell. Therefore, an increase of the steam partial pressure means a pressure increase in the primary containment vessel at the time of an accident, and the strength of the primary containment vessel must be increased in order to withstand this pressure.
Alternatively, it is necessary to increase a heat transfer area of the heat exchanger mounted in the cooling water pool provided at the upper portion of the primary containment vessel so that the uncondensed steam which will cause the steam partial pressure to increase will not flow into the suppression pool. In this case, in conformity with the increase of the heat transfer area of the heat exchanger, the capacity of the cooling water pool disposed at the high level must be increased, and the strength of the primary containment vessel must be increased with a view to increasing a seismic strength.
Either of the above measures results in an increased strength of the primary containment vessel, and the structure needs to be increased in strength, for example, by increasing the thickness of the wall of the containment vessel, and therefore the cost of the plant is increased.